Speed of light

Basic idea

$c$ is the invariant speed of causality and the maximum signal speed. Crucially: only the two-way (round-trip) speed is operationally measured; the one-way speed depends on a synchronisation convention.

Key formulas

• $c \approx 2.998 \times 10^8$ m/s (exact since 1983: $c = 299{,}792{,}458$ m/s)
• $c = 1/\sqrt{\mu_0 \varepsilon_0}$ (from Maxwell’s equations)
• Two-way speed (Einstein convention): $\bar c = 2 L / (t_{A\to B} + t_{B\to A}) = c$

Speed of light

The speed of light is c, but c is measured through entire return trip from point A to point B and back again. It simply is not possible to have a one way measurement of the speed of light.

This means that the following scenario could happen: Lets define some variables: c' := c/2

time(A,B) = c' time(B,A) = ∞

Since the total return time equals to c, this is perfectly fine. Physics does not break under the above scenario.

Siblings

• Relativity